42 research outputs found
Disorder-induced double resonant Raman process in graphene
An analytical study is presented of the double resonant Raman scattering
process in graphene, responsible for the D and D features in the
Raman spectra. This work yields analytical expressions for the D and
D integrated Raman intensities that explicitly show the dependencies
on laser energy, defect concentration, and electronic lifetime. Good agreement
is obtained between the analytical results and experimental measurements on
samples with increasing defect concentrations and at various laser excitation
energies. The use of Raman spectroscopy to identify the nature of defects is
discussed. Comparison between the models for the edge-induced and the
disorder-induced D band intensity suggests that edges or grain boundaries can
be distinguished from disorder by the different dependence of their Raman
intensity on laser excitation energy. Similarly, the type of disorder can
potentially be identified not only by the intensity ratio
, but also by its laser energy
dependence. Also discussed is a quantitative analysis of quantum interference
effects of the graphene wavefunctions, which determine the most important
phonon wavevectors and scattering processes responsible for the D and
D bands.Comment: 10 pages, 4 figure
A Magnon Scattering Platform
Scattering experiments have revolutionized our understanding of nature.
Examples include the discovery of the nucleus, crystallography, and the
discovery of the double helix structure of DNA. Scattering techniques differ by
the type of the particles used, the interaction these particles have with
target materials and the range of wavelengths used. Here, we demonstrate a new
2-dimensional table-top scattering platform for exploring magnetic properties
of materials on mesoscopic length scales. Long lived, coherent magnonic
excitations are generated in a thin film of YIG and scattered off a magnetic
target deposited on its surface. The scattered waves are then recorded using a
scanning NV center magnetometer that allows sub-wavelength imaging and
operation under conditions ranging from cryogenic to ambient environment. While
most scattering platforms measure only the intensity of the scattered waves,
our imaging method allows for spatial determination of both amplitude and phase
of the scattered waves thereby allowing for a systematic reconstruction of the
target scattering potential. Our experimental results are consistent with
theoretical predictions for such a geometry and reveal several unusual features
of the magnetic response of the target, including suppression near the target
edges and gradient in the direction perpendicular to the direction of surface
wave propagation. Our results establish magnon scattering experiments as a new
platform for studying correlated many-body systems
Supporting Spartina: Interdisciplinary Perspective Shows Spartina As A Distinct Solid Genus
In 2014, a DNA-based phylogenetic study confirming the paraphyly of the grass subtribe Sporobolinae proposed the creation of a large monophyletic genus Sporobolus, including (among others) species previously included in the genera Spartina, Calamovilfa, and Sporobolus. Spartina species have contributed substantially (and continue contributing) to our knowledge in multiple disciplines, including ecology, evolutionary biology, molecular biology, biogeography, experimental ecology, biological invasions, environmental management, restoration ecology, history, economics, and sociology. There is no rationale so compelling to subsume the name Spartina as a subgenus that could rival the striking, global iconic history and use of the name Spartina for over 200 yr. We do not agree with the subjective arguments underlying the proposal to change Spartina to Sporobolus. We understand the importance of both the objective phylogenetic insights and of the subjective formalized nomenclature and hope that by opening this debate we will encourage positive feedback that will strengthen taxonomic decisions with an interdisciplinary perspective. We consider that the strongly distinct, monophyletic clade Spartina should simply and efficiently be treated as the genus Spartina
Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure
Ultrafast electron thermalization - the process leading to Auger
recombination, carrier multiplication via impact ionization and hot carrier
luminescence - occurs when optically excited electrons in a material undergo
rapid electron-electron scattering to redistribute excess energy and reach
electronic thermal equilibrium. Due to extremely short time and length scales,
the measurement and manipulation of electron thermalization in nanoscale
devices remains challenging even with the most advanced ultrafast laser
techniques. Here, we overcome this challenge by leveraging the atomic thinness
of two-dimensional van der Waals (vdW) materials in order to introduce a highly
tunable electron transfer pathway that directly competes with electron
thermalization. We realize this scheme in a graphene-boron nitride-graphene
(G-BN-G) vdW heterostructure, through which optically excited carriers are
transported from one graphene layer to the other. By applying an interlayer
bias voltage or varying the excitation photon energy, interlayer carrier
transport can be controlled to occur faster or slower than the intralayer
scattering events, thus effectively tuning the electron thermalization pathways
in graphene. Our findings, which demonstrate a novel means to probe and
directly modulate electron energy transport in nanoscale materials, represent
an important step toward designing and implementing novel optoelectronic and
energy-harvesting devices with tailored microscopic properties.Comment: Accepted to Nature Physic
The dynamic DNA methylomes of double-stranded DNA viruses associated with human cancer
The natural history of cancers associated with virus exposure is intriguing, since only a minority of human tissues infected with these viruses inevitably progress to cancer. However, the molecular reasons why the infection is controlled or instead progresses to subsequent stages of tumorigenesis are largely unknown. In this article, we provide the first complete DNA methylomes of double-stranded DNA viruses associated with human cancer that might provide important clues to help us understand the described process. Using bisulfite genomic sequencing of multiple clones, we have obtained the DNA methylation status of every CpG dinucleotide in the genome of the Human Papilloma Viruses 16 and 18 and Human Hepatitis B Virus, and in all the transcription start sites of the Epstein-Barr Virus. These viruses are associated with infectious diseases (such as hepatitis B and infectious mononucleosis) and the development of human tumors (cervical, hepatic, and nasopharyngeal cancers, and lymphoma), and are responsible for 1 million deaths worldwide every year. The DNA methylomes presented provide evidence of the dynamic nature of the epigenome in contrast to the genome. We observed that the DNA methylome of these viruses evolves from an unmethylated to a highly methylated genome in association with the progression of the disease, from asymptomatic healthy carriers, through chronically infected tissues and pre-malignant lesions, to the full-blown invasive tumor. The observed DNA methylation changes have a major functional impact on the biological behavior of the viruses
HIV-1 Vpu Protein Mediates the Transport of Potassium in Saccharomyces cerevisiae
Human immunodeficiency virus type 1 (HIV-1) Vpu is an integral membrane protein that belongs to the viroporin family. Viroporins interact with cell membranes, triggering membrane permeabilization and promoting release of viral particles. In vitro electrophysiological methods have revealed changes in membrane ion currents when Vpu is present; however, in vivo the molecular mechanism of Vpu at the plasma membrane is still uncertain. We used the yeast Saccharomyces cerevisiae as a genetic model system to analyze how Vpu ion channel impacts cellular homeostasis. Inducible expression of Vpu impaired cell growth, suggesting that this viral protein is toxic to yeast cultures. This toxicity decreased with extracellular acidic pH. Also, Vpu toxicity diminished as the extracellular K(+) concentration was increased. However, expression of the Vpu protein suppresses the growth defect of K(+) uptake-deficient yeast (Δtrk1,2). The phenotype rescue of these highly hyperpolarized cells was almost total when they were grown in medium supplemented with high concentrations of KCl (100 mM) at pH 7.0 but was significantly reduced when the extracellular K(+) concentration or pH was decreased. These results indicate that Vpu has the ability to modify K(+) transport in both yeast strains. Here, we show also that Vpu confers tolerance to the aminoglycoside antibiotic hygromycin B in Δtrk1,2 yeast. Our results suggest that Vpu interferes with cell growth of wild-type yeast but improves proliferation of the hyperpolarized trk1,2 mutant by inducing plasma membrane depolarization. Furthermore, evaluation of the ion channel activity of the Vpu protein in Δtrk1,2 yeast could aid in the development of a high-throughput screening assay for molecules that target the retroviral protein.This study was supported by Grants PI PI05/00013 and PI08/0912 from Fondo de Investigación Sanitaria. L.H. and N.M. were holders of Predoctoral Fellowships from Instituto de Salud Carlos III.S
Electrochemical half-reaction-assisted sub-bandgap photon sensing in a graphene hybrid phsotodetector
The photogating effect has been previously utilized to realize ultra-high photoresponsivity in a semiconductor-graphene hybrid photodetector. However, the spectral response of the graphene hybrid photodetector was limited by the bandgap of the incorporated semiconductor, which partially compromised the broadband absorption of graphene. Here, we show that this limitation can be overcome in principle by harnessing the electron-accepting ability of the electrochemical half-reaction. In our new graphene phototransistor, the electrochemical half-reaction serves as an effective reversible electron reservoir to accept the photoexcited hot electron from graphene, which promotes the sub-bandgap photosensitivity in a silver chloride (AgCl)-graphene photodetector. The photoconductive gain of ~ 3 × 109 electrons per photon in the AgCl-graphene hybrid is favored by the long lifetime of photoexcited carriers in the chemically reversible redox couple of AgCl/Ag0, enabling a significant visible light (400–600 nm) responsivity that is far beyond the band-edge absorption of AgCl. This work not only presents a new strategy to achieve an electrically tunable sub-bandgap photoresponse in semiconductor-graphene heterostructures but also provides opportunities for utilizing the electrochemical half reaction in other two-dimensional systems and optoelectronic devices.published_or_final_versio
Molecular dynamics and Monte Carlo simulations of the sputtering of a nanoporous solid
We calculate the sputtering induced on a nanoporous material by fast penetrating ions, such as those used for track formation and surface modification, in order to better understand and quantify the ejection and redeposition of atoms in open cell nanofoams. We model the ion-induced sputtering yield from a porous solid using a Monte Carlo approach and compare the results for the sputtering yields, angular and depth distributions of the ejecta, with Molecular dynamics simulations and find good agreement. For certain foam geometries, our simulations predict enhanced sputtering yields compared to the yields from a fully dense solid.Fil: Rodriguez Nieva, J. F.. ComisiĂłn Nacional de EnergĂa AtĂłmica. Gerencia del Area de EnergĂa Nuclear. Instituto Balseiro; ArgentinaFil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Mendoza; Argentina. Universidad Nacional de Cuyo; Argentin